eAccelerator is a free open source PHP  accelerator,  optimizer,
encoder  and dynamic content cache for PHP. It increases  performance
of  PHP  scripts  by caching them in compiled state, so that the
overhead of  compiling  is  almost completely eliminated. Also it
uses some optimizations to speed  up  execution of PHP scripts.
eAccelerator typically reduces server load and  increases  the
speed of your PHP code by 1-10 times.

eAccelerator is a fork of TurckMMCache 
( http://sourceforge.net/project/turckmm-cache/  by Dmitry Stogov )

eAccelerator stores compiled PHP scripts in shared memory  and
executes  code directly from it. It creates  locks  only  for
short  time,  while  searching compiled PHP script in the cache,
so one script can be executed simultaneously by several engines.
Files that can't fit in shared memory are cached  on  disk only.